pyxis: Adjust BOARD_SYSTEMIMAGE_PARTITION_RESERVED_SIZE

Many pyxis users flashes stock camera that has to be placed in system partition.
After I444ce2fcd37cdfa58d83a7e872dc547fb05a2a89 the partition is too small.

Change-Id: Id75b04f5b778a60ed01e7f6def87442dee193869

BoardConfig.mk

@@ -25,6 +25,7 @@ TARGET_KERNEL_CONFIG += xiaomi/pyxis.config
 
 # Partitions
 BOARD_BOOTIMAGE_PARTITION_SIZE := 134217728
+BOARD_SYSTEMIMAGE_PARTITION_RESERVED_SIZE := 536870912
 
 # Properties
 TARGET_SYSTEM_PROP += $(DEVICE_PATH)/system.prop